Skin growth, repair and healing are subject to complex biological control mechanisms which act via both positive and negative signals. Such signals act at the level of controlling cell proliferation, differentiation and migration, and are typically mediated by growth factor polypeptides. In this regard, important growth factors include epidermal growth factor (EGF) and insulin-like growth factors (IGF-I and -II).
Human IGF-I has been reported to exert a wide range of biological activities including stimulation of cell proliferation, differentiation and migration, protection from protein degradation and apoptosis, as well as regulation of endocrine factors such as growth hormone. IGF-II has similar properties to IGF-I but appears to be more relevant to carcinogenesis and fetal and embryonic development, IGF-I having a greater role in postnatal development.
Both IGF-I and IGF-II act through a binding interaction with the type I IGF receptor (IGFR). The availability of the IGFs for such an interaction is regulated by insulin like growth factor binding proteins (IGFBPs 1-6). IGFBPs are known to both positively and negatively regulate IGF function as well as exhibit IGF-independent activity.
Another functional component of IGF pathways is the type II IGFR which is also known as the cation-independent mannose-6-phosphate receptor (CI-MPR). The type II IGFR is a multifunctional protein that binds lysosomal enzymes bearing mannose-6-phosphate moieties, as well as IGF-II, although the functional significance of IGF-II binding is unclear (O'Dell & Day, 1998, Int. J. Biochem. Cell Biol. 30 767; Braulke, 1999, Horm. Metab. Res. 31 242; Nykjaer et al., 1998, J. Cell. Biol. 141 815).
The IGFs have also been reported to bind another group of proteins termed “IGFBP-related proteins” which share structural similarity and include connective tissue growth factor (CTGF) and products encoded by the mac25, nov and cyr61 genes. These bind IGFs with much lower affinity than do IGFBPs.
More recently, vitronectin (VN) has been identified as an extracellular matrix protein, structurally unrelated to IGFBPs and IGFBP-related proteins, that binds IGF-II but not IGF-I (Upton et al., 1999, Endocrinol. 140 2928).
Vitronectin is an ˜75 kD, glycosylated extracellular matrix protein which is also found in blood, and has been implicated in cancers, bone diseases and pathological disorders involving angiogenesis (reviewed in Schvartz et al., 1999, Int. J. Biochem. Cell Biol. 31 539). The role of vitronectin in events such as angiogenesis and tumorigenesis at least partly resides in the ability of vitronectin to bind integrins and to interact with components of the urokinase plasminogen activator system (for example PAI-1, uPAR, plasminogen) to thereby promote cell proliferation, adhesion, spreading and migration. Vitronectin has more specifically been implicated in preventing tumor cell apoptosis in response to drug treatment (Uhm et al., 1999, Clin. Cancer Res. 5 1587). Vitronectin appears to be a carrier of IGF-II in the circulation (McMurtry et al., 1996, J. Endocrinol. 150 149).